If it still slips after you have thoroughly cleaned it, then it might be time to get a new one. I have a gss clamp and have had no problems with it. I usually clean it after about 5 string jobs. The gss clamp is also the one master racket technician, yulitle, recommends and he does not steer you wrong when it comes to stringing.

Sounds to me like the clamp is just very dirty. I would take the springs out and take it apart and clean it very good with a tooth brush and alcohol. It is hard to really clean a clamp with a pipe cleaner or a shoe string.

Most issues with starting clamps is stringers do not clean them like the machines clamps. Like said, just push in the springs and take the springs off, then it is easier to clean the holding surface, then replace the springs and you should be all set.